1. Field of the Invention
The present invention relates to electronic circuits and, more specifically, to bandgap voltage reference circuits.
2. Description of the Related Art
Bandgap voltage regulators are typically used to provide substantially constant reference voltages for circuits that operate in environments subject to temperature fluctuation. Generally, bandgap circuits develop a voltage that consists of a summation of a base emitter voltage and a voltage proportional to the difference between the base-to-emitter voltages, ΔVBE, of two bipolar transistors. This difference is linear with temperature and has a certain positive temperature coefficient +TCΔVBE. On the other hand the base emitter voltage VBE of a bipolar transistor has a negative temperature coefficient −TCVBE. By proper scaling of the ΔVBE and adding it to a VBE, a voltage results that has a zero temperature coefficient. Because TCΔVBE is smaller than TCVBE, the ΔVBE needs to be scaled (amplified) to cancel the TCVBE. A disadvantage of amplifying ΔVBE is that circuit noise is also amplified.
FIG. 1 depicts a prior art circuit 100 for amplifying ΔVBE to create a bandgap reference circuit. The circuit 100 is comprised of four transistors. Two transistors M1 and M2 form a current mirror, forcing the collector currents of the bipolar transistors Q1 and Q2 to be equal. The transistors Q1 and Q2 generate the voltage difference ΔVBE across R1 equal to (kT/q)*ln(M) where M is the ratio in emitter area between Q2 and Q1. The ratio between resistors R1 and R2 determines the scaling factor of ΔVBE. The output voltage is the sum of the scaled ΔVBE and the base emitter voltage VBE of Q1. More specifically, the power supply VDD (e.g., 3.3 volts) is connected to the source terminals of transistors M1 and M2. Transistor M1 has a drain terminal connected to the collector and base terminal of transistor Q1 and the emitter terminal of transistor Q1 is connected to ground through resistor R2. The gate and drain terminals of transistor M2 are connected to one another and the gate terminals of transistors M1 and M2 are connected to one another. The drain and gate terminal of transistor M2 are connected to the collector terminal of transistor Q2 and the emitter terminal of transistor Q2 is connected to ground through both resistors R1 and R2. In this manner, transistors M1 and M2 form a current mirror and transistors Q1 and Q2 generate the voltage difference ΔVBE. For this circuit, the bandgap voltage is given by Vbandgap=VBE+ΔVBE·(R2/R1). Due to the multiplication of ΔVBE,the noise of resistor R1 is multiplied at the output such that its noise power contribution is equivalent to
      4    ·    k    ·    T    ·                            R          1                ⁡                  (                                    R              2                                      R              1                                )                    2        ,where k is the Boltzmann constant, T is temperature and R1 and R2 are the resistance values of resistors R1 and R2.
As can be seen by the noise equation, the classic bandgap circuit 100 is very noisy. By reducing the impedance level of resistors R1 and R2 the level of noise can be reduced, but the power consumption of the circuit increases.
In other attempts to reduce the noise of the bandgap circuit, the ΔVBE values of the transistor combinations are stacked to reduce the amount of amplification needed to obtain a reference voltage. Stacking transistors reduces the amplification needed in each amplification stage and thus reduces noise level in the output signal. In the stacked transistor circuit, the ΔVBE values of each transistor combination add directly to one another, while the noise adds on a power basis. Since power is proportional to voltage squared, the ratio of the output voltage (after amplification) to noise voltage decreases by the square root of the number of stacked ΔVBE values. In one known realization, U.S. Pat. No. 6,288,525, the stacked transistor circuit uses both NPN and PNP transistors as well as an operational amplifier. As such, these circuits are less noisy than traditional bandgap circuits, but they are significantly more complex. A further reduction in noise and complexity can be achieved with the present invention.
Therefore there is a need in the art for a low noise bandgap circuit having a relatively simple structure.